The invention relates to a slot-drilling cutter having cutting plates adapted for high metal cutting capacity.
Metal cutting tools are used for cutting while a rotating tool is axially advanced. In this process, the cutting edges of the metal cutting tool are in constant engagement with the material of the workpiece and perform an essentially uninterrupted cut. Because of this, the cutting stress on the cutting edges at the end of the tool is steady, i.e. not vibrating or pulsating. However, it is often also desirable that the metal cutting tool be laterally advanced for milling cutting. Peripheral cutting edges are required for this. Since the peripheral cutting edges in this case are not in steady, but rather in intermittent, contact with the workpiece cutting is interrupted, and vibrating or pulsating forces are accordingly created, which act on the peripheral cutting edges. The tool suitable for such work must be able to withstand such forces.
A machining tool is known from EP 0 257 372, which is made completely of a hard alloy and has been molded in an extruding process. It is designed to be dynamically balanced in respect to its axis of rotation and has two end cutting edges and two peripheral cutting edges. When a cutting edge is worn, the cutting tool as a whole must be replaced or refitted.
A slot-drilling cutter is known from DE 34 17 168 A1, whose tool body is equipped with cutting plates at the tool body end, as well as on the circumference. One of the two cutting plates which are active at the tool body end is arranged as a lateral plate. The cutting edge of this cutting plate extends radially outward, starting at the center of rotation, but not over the entire radius. The end cutting edge of the other complementary cutting plate arranged at the tool end extends from an outer corner area radially inward, but not as far as the lateral plate. Thus there are areas of the radius in which only one cutting edge is active. This applies in particular to the exterior area of the slot-drilling cutter. The axial advancement of the slot-drilling cutter is therefore limited by the metal cutting yield of this one cutting edge.
The tool body of this known slot-drilling cutter moreover has plate seats for peripheral cutting plates. These are arranged in two rows, which are offset in respect to each other and are spaced apart. Here, the cutting edges of the peripheral cutting plates in the two rows overlap each other in such a way that areas exist, in which two cutting edges are active, wherein in the gaps between two peripheral cutting plates of a row only a cutting edge section of a cutting plate of the other row is active. Because of this, the advancement speed for lateral advancement movements is limited by the metal cutting yield of this cutting edge area.
Another slot-drilling cutter is known from DE 196 09 820. Its tool body has several chip grooves with plate seats. These are arranged in such a way that the cutting plates of the individual rows assume different axial positions and therefore overlap at the peripheral cutting edges. At the corners, i.e. in the transition area from the peripheral cutting edges to the end cutting edges, two round plates have been inserted. These are arranged in different axial and radial positions, so that one of them is active with its peripheral cutting edge, while the other is active with its end cutting edge. Thus, the round plates limit the metal cutting yield for the drilling movement (axial advancement), as well as for the milling movement (radial advancement).
Limited metal cutting yields result with the drilling tools, or drilling-milling tools, in accordance with the prior art mentioned above. If, however, the tools are not equipped with cutting plates, but are made completely from a hard alloy, maintenance of worn cutting edges is impossible, or at least expensive.
A milling head is furthermore known from EP 0 537 476 B1, which is also equipped for drilling. The milling head has an elongated slim tool body, at whose end two round plates are arranged as cutting plates. In this case these cutting plates are maintained in appropriate seats in such a way that their cutting edges touch each other at the axis of rotation, or that the cutting plates overlap, i.e. partially touch each other at the metal cutting faces. Thus, the metal cutting faces (front ends) of the cutting plates are located on a common plane. In order to obtain a sufficient clearance angle, the cutting plates must have significant conical shape, in particular with drilling-milling tools of small diameter. This weakens the cutting edges.
Based on the foregoing, it is an object of the invention to provide a slot-drilling cutter which is versatile in use, produces a large metal cutting yield, permits a precise metal cutting of the workpiece, and which is simple to maintain.
A slot-drilling cutter in accordance with the invention is equipped with cutting plates, which are releasably secured on a tool body. Suitable clamping means, such as clamping claws, clamping screws, fastening screws or the like must be used for fastening. It is particularly advantageous if the cutting plates have a central fastening opening, through which a fastening screw is to extend. The latter presses the cutting plate against the plate seat and, if required, against an appropriate lateral contact.
A special feature of the slot-drilling cutter in accordance with the invention is that the tool body has two plate seats at its end, which are arranged symmetrically in respect to the axis of rotation and are provided for cutting plates which-are substantially identical. The cutting plates respectively have at least an end cutting edge, which is oriented substantially radially with respect to the axis of rotation, a peripheral cutting edge, which is oriented substantially along the longitudinal axis of the tool, and a corner cutting edge, which constitutes a transition between the end cutting edge and the peripheral cutting edge. The end cutting edges of the two cutting plates are not offset against each other in the radial direction, instead they are arranged in the same axial position. The inclinations of the end cutting edges furthermore are similar. Because of this, a fixed point on one end cutting edge defines the same pitch circle as the corresponding point on the other end cutting edge fixed at the same radius. In other words, both end cutting edges lie in a common plane. The end cutting edges are essentially straight, i.e. they can be slightly curved, as long as the radius of curvature exceeds half the plate diameter. The end cutting edge also can be divided into several sections, which together enclose an obtuse angle. The end cutting edge furthermore can have one or several steps.
The cutting plates are arranged in such a way that their front ends, i.e. the surface whose normal surface line points in the circumferential direction, are offset in respect to each other. Thus, the front ends of the cutting plates are respectively contained on different planes which are parallel with each other and do not extend through the axis of rotation. In regard to the rotatory movement of the metal cutting tool, the respective plane in which the front end of the respective cutting plate is located, leads a radial plane with which it is parallel and which extends through the axis of rotation. The lead of the peripheral cutting edges in this way results a clearance angle without the respective cutting plates having to have a special conicity. Thus, the cutting plates can have a right angle, or an only slightly more acute angle, constituting the lip angle at their peripheral cutting edges. This assures the stability of the cutting edge and in particular enables a high metal cutting yield.
Furthermore, this arrangement permits the support of the cutting plates toward the inside against a narrow, and therefore rigid strip, or the direct contact of the lateral faces of the cutting plates against each other. This type of support permits the transmission of radial forces occurring in the course of milling. The mentioned offset of the top surfaces of the cutting edges against each other therefore provides the basis for an improved lateral support of the cutting plates. The improved support of the cutting edges in turn can be used for increased metal cutting yield.
Finally, the same applies to the corner cutting edges and the peripheral cutting edges. Because of this, respectively two cutting edges are active during drilling operations (axial advancement), as well as during milling operations (radial advancement). The metal cutting yield is therefore doubled in comparison with a tool with only one cutting plate, and the same advancement speeds can be employed for drilling, as well as milling. This considerably simplifies machine programming, because the danger of overloading the tool because of inadvertent excessive advancement speeds, or because of unacceptably long treatment times for safety reasons, can be prevented.
The cutting plates are arranged on the tool body offset by 180xc2x0 in relation to the axis of rotation, and therefore symmetrically in respect to the axis of rotation. Because of this, the forces generated by the two cutting plates during drilling operations essentially cancel each other out so that no significant radial forces are created. This allows a high degree of precision during the treatment operations, in particular during drilling.
In principle, the end cutting edges, corner cutting edges and peripheral cutting edges of the two cutting plates can have lengths differing from each other. In that case, if care is taken, cutting edge sections which do not have an assured complete overlap of the two cutting plates, either are not used for metal cutting, or are assisted by other cutting plates. But it is preferred to embody end cutting edges, corner cutting edges and peripheral cutting edges each with the same length and in the same shape. In this way the cutting plates arranged on the end of the tool body can be identical, which lowers the outlay for maintenance and production.
The end cutting edges and the peripheral cutting edges are designed to be essentially straight. But the corner cutting edge, depending on the requirements, may be curved in an arc to a greater or lesser extent. Starting at the corner cutting edge, both end cutting edges extend inward at the same length toward the center of rotation surrounding the axis of rotation. The center of rotation not covered by the cutting edges forms an area which is not treated by milling during drilling, in which a small peg can initially remain on the workpiece. This is milled off or broken off as a whole during the subsequent lateral movement of the drilling tool (milling).
The end cutting edges are preferably inwardly inclined, i.e. their end near the axis of rotation is offset in the direction of the shaft of the tool body in respect to the corner cutting edge. In other words, the sections of the end cutting edges adjoining the corner cutting edge project axially the most. In this way, the working of level surfaces during lateral advance (milling) is made possible, and the drilling tool can be employed simultaneously as slot cutter (slot-drilling cutter). Thus, the two end cutting edges define a truncated cone which constitutes an angle of taper of preferably greater than 170xc2x0. But the peripheral cutting edges are preferably arranged on a cylinder surface. If the peripheral cutting edges are oriented parallel with the axis of rotation, the peripheral cutting edges can be designed to be completely straight. But if the cutting plates are installed at an angle to the axis in the tool body, or if an axial milling angle is provided on the cutting plates (possibly because the cutting plates are designed to be wedge-shaped), the peripheral cutting edges are curved in such a way that they lie on the generated surface of an imagined cylinder. If required, the cutting edges can be helical, in the shape of an elliptical arc, or follow any other curve located on the imagined generated surface of the cylinder.
On its side facing the axis of rotation, the end cutting edge preferably makes a transition into a section which is inclined in respect to the remaining end cutting edge and which herein is called an inner cutting edge. The end cutting edges are respectively arranged in front of a radius which is parallel with them. This provides a high cutting plate stability of the cutting plates in the area of the peripheral cutting edges possible. In that case the inner cutting edges then lead behind this radial line in the vicinity of the axis of rotation (area of the center of rotation), which also makes metal cutting work possible there. Possibly remaining pegs are therefore slim and do not hamper the drilling advancement.
For absorbing the radial contact forces occurring in the course of milling, the two cutting plates of the metal cutting tool can be in contact with a strip area of the tool body which contains the axis of rotation and is embodied as a wedge, for example, and tapers narrowly toward the front end of the tool body. This makes it possible to bring the end cutting edges directly up to the axis of rotation and to minimize to a very small area the center of rotation not covered by the metal cutting process. If necessary, the plate seats can also be designed to be open toward the inside, i.e. the cutting plates will not find a radial lateral contact face at the plate seats. In this case it is possible that the cutting plates meet at the axis of rotation and together define a narrow gap, or are pressed against each other. In this case the inner cutting edges, i.e. the sections of the end cutting edges extending into the center of rotation, are preferably inclined toward the plate seat so that these cutting edge sections do not run in front of, but behind, the center of rotation. In this way cutting edge sections are prevented from being moved opposite their orientation.
It is possible to provide more than two cutting plates on the end of the tool body. Here, too, it is possible to let the cutting plates contact directly in order to form a gap-free cutting edge. This increases the metal cutting yield and makes simple plate seats possible.
A longitudinal step can be used for supporting the cutting plates in the lateral direction (radially), a transverse step can be provided in the axial direction. In addition, a dowel screw can be provided, which provides the support in one direction. The metal cutting tool is preferably a slot-drilling cutter which can be used for drilling, as well as milling. An axial entry into the workpiece is possible because of the complete outfitting of the end of the drilling tool with cutting plates.